A single-stage switching power converter such as a flyback converter cycles a power switch as controlled by a controller to deliver power to a load. At startup for a flyback converter operating, for example, to charge a discharged battery (full load condition), the controller cycles the power switch such that the output current and output voltage both begin to rise from their initial zero values. But the output current cannot exceed a maximum output current or components such as the power switch may be damaged. Upon reaching the maximum output current limit, the controller will thus switch into a constant current mode of operation that maintains the output current at the maximum current limit. FIG. 1 illustrates the output voltage during operation in a constant current mode 100 for a flyback converter. As the battery is progressively charged, the output voltage in constant current mode 100 will gradually rise until a desired output voltage (Vdesired) is reached at a transition point 110, whereupon the controller transitions to operate in a constant voltage mode 105. Since the controller is driving a full load, it regulates the cycling of the power switch during constant voltage mode 105 using pulse width modulation at a constant switching frequency.
As the load reduces (e.g., as a battery is charged), the output current continues to reduce in constant voltage mode 105 in conjunction with a reduction of the duty cycle for the pulse width modulation. But even with the pulse width progressively reduced, pulse width modulation operation will eventually overdrive the load as the load continues to reduce. The controller will thus transition into a constant voltage pulse frequency mode for light load operation as shown in FIG. 2, which illustrates the switching frequency as a function of the output load. In particular, the switching power converter will transition from the pulse width modulation (PWM) mode of operation into the pulse frequency mode (PFM) of operation at a predetermined load value that equals some fraction of the maximum load value. While operating in the PWM mode, the controller maintains the switching frequency at a maximum value Fmax. But in the PFM mode, the controller will gradually reduce from this maximum value as the load is decreased.
Although the pulse frequency modulation mode enables the controller to increase efficiency without overdriving the load during low load conditions, the controller operates in the constant voltage mode. Thus, there is a need in the art for additional control modes of operation during low load conditions.
Embodiments of the present disclosure and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.